Brainer, You need to know the history of "Don't ask, don't tell" so that you don't cause offence by inadvertent misuse of the phrase. Here is a summary excerpted from the full Wikipedia article. 1. "Don't ask, don't tell" (DADT) was the official United States policy on service by gays and lesbians in the military instituted by theClinton Administration on February 28, 1994. 2. The "don't ask" part prohibited superiors from asking about or investigating the sexual orientation of their staff. 3. The "don't tell" part prohibited everyone within the armed forces to talk about their own bisexual or homosexual inclinations and activities. 4. The practical effect of this policy was that military personnel were prohibited from discriminating against or harassing closeted homosexual or bisexual service members or applicants, while barring openly gay, lesbian, or bisexual persons from military service. 5. In December, 2010, legislation was passed by Congress to end DADT, subject to a certain certification by the President and the Chief of Staff that ending it would not harm U.S. military readiness. 6. In July, 2011, a ruling from a federal appeals court barred further enforcement of the U.S. military's ban on openly gay service members. 7. In July, 2011, President Barack Obama, Secretary of Defense Leon Panetta, and Chairman of the Joint Chiefs of Staff Admiral Mike Mullen gave that certification. DADT officially ended.

They're trying to be secretive. Either they know the answer and just won't tell you because it's shady or they don't know the answer and are telling you not to ask because it's probably something you don't want to know -again for shady or mysterious reasons.

Really, this depends on the culture you're in. In mine, England, it bears connotations of homosexual relations, although not everyone is aware of it! But given the context, it could be used in the sense of "it's a secret".